Valence Fluctuation and the Temperature Dependence of Copper Nuclear Quadrupole Interaction in Eu${\mathrm{Cu}}_2$${\mathrm{Si}}_2$ and Yb${\mathrm{Cu}}_2$${\mathrm{Si}}_2$

The quadrupole interaction of copper has been measured as a function of temperature in Eu${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ and Yb${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$. These measurements suggest that in these systems Hirst's "interconfiguration fluctuation" model provides a better description of the valence fluctuation than the localized-electron model. The observation of the influence of the valence-fluctuation phenomenon on the quadrupole interaction is reported for the first time and the importance of the NMR measurements on these materials is pointed out.